1. Field of the Invention
The present invention relates in general to a circuit for driving a display device in which at least associated ones of display elements emit light by causing a D.C. current to flow through the at least associated ones of display elements, and more particularly to a circuit for measuring the voltage to current characteristics of the at least associated ones of display elements emitting the light to correct the driving state thereof in order to stabilize the luminance of the at least associated ones of the display elements emitting the light.
2. Description of the Related Art
The construction of a conventional circuit for driving a light emitting elements display device is shown in FIG. 5. In addition, FIG. 6 is a plan view showing an element shape of the conventional light emitting elements display device. In FIG. 5, display elements 501 to 520 are electrically connected in a matrix to one another; an output terminal of a constant current source 521 which has a switching function and from which a constant current is caused to flow is electrically connected to a positive electrode side of the display elements 501, 506, 511 and 516, respectively; an output terminal of a constant current source 522 is electrically connected to display elements 502, 507, 512 and 517, respectively; an output terminal of a constant current source 523 is electrically connected to display elements 503, 508, 513 and 518, respectively; an output terminal of a constant current source 524 is electrically connected to display elements 504, 509, 514 and 519, respectively; and an output terminal of a constant current source. 525 is electrically connected to display elements 505, 510, 515 and 520, respectively. On the other hand, a switch 526 through which a current is caused to flow into the ground is electrically connected to a negative electrode side of the display elements 501, 502, 503, 504 and 505, respectively; a switch 527 is electrically connected to the display elements 506, 507, 508, 509 and 510; a switch 528 is electrically connected to the display elements 511, 512, 513, 514 and 515, respectively; and a switch 529 is electrically connected to the display elements 516, 517, 518, 519 and 520, respectively.
As shown in FIG. 6, in a prior art example, the areas of the display elements 501 to 520 which are arranged in a light emitting elements display device 601 are identical to one another, and also the constant current sources 521 to 525 supply the display elements 501 to 520 with the currents which are equal to one another. Therefore, all of the display elements have the same current density. Many D.C. driven self-light emitting devices have the characteristics in which the amount of light emission is roughly proportional to the current, and also have the V-I characteristics which are not linear as in a resistor and which are changed due to the long term degradation and temperature changes. For this reason, in order that a constant current may be caused to flow through the associated one of the display elements, the constant voltage drive is not required, but the constant current drive is required, and hence in general, the constant current drive is carried out to suppress the fluctuation of brightness.
In addition, each of the constant current sources 521 to 525 has a switching function in order to turn ON or OFF the associated one of the display elements and hence can turn ON or OFF the associated one of the constant current sources. On the other hand, the switches 526 to 529 are successively turned ON one by one in a time sharing manner. Thus, a plurality of switches are not turned ON at the same time. For example, in order to light the display element 501, the constant current source 521 and the switch 526 are both turned ON. Likewise, all of the display elements can be selectively lighted on the basis of the combination of the constant current sources 521 to 525 and the switches 526 to 529.
By adopting the construction as described above, a large number of light emitting elements can be arranged to carry out the dot matrix display, which results in various displays being able to be carried out.
If a conventional display device and a driving circuit are employed for a watch for which the miniaturization and the low power are both required, then an extra power source voltage is required in the normal constant current drive, and also the electric power which is obtained by multiplying a difference between the power source voltage and the voltage applied to a load by a load current is lost in the form of calorification. In addition thereto, there is a limit to the size of the battery as in a watch. Therefore, since the multiplying factor for boosting the power source must be increased in terms of a circuit in the apparatus for which there is limit to the power source, this is disadvantageous in terms of the power consumption and the circuit scale.
In addition, while in order that fine characters may be displayed, the fine dot matrix display needs to be employed, and the case where the segment display is employed is more advantageous than the former in terms of the power consumption. However, in the case of the segment display, it is difficult to make the areas of the segments identical to one another. For this reason, when the display elements corresponding to the segments are driven by the constant current sources having the same current value, the current density is changed in dependence on the segments, which causes the difference in the luminance of the segments.
For the purpose of reducing the difference in the luminance of the segments, it is considered that the display elements corresponding to the segments are driven by the constant voltage drive. As a result, though the initial luminance difference can be reduced, the current is greatly changed due to the long term degradation and temperature changes and hence the luminance change is caused. Therefore, the constant voltage drive can not be adopted as long as the correction is carried out for the current.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore an object of the present invention to realize a circuit for driving a light emitting elements display device which is excellent in display quality with low power by a reducing the luminance change due to the change in the characteristics, caused by long term degradation and temperature changes, of the light emitting elements display device while obtaining the high utilization factor for a power source by driving the light emitting elements display device a constant voltage.
By utilizing the fact that each of the display elements is dynamically driven and a driving FET is repeatedly turned ON or OFF in order to obtain the stable light emission having less luminance change, a current which is being caused to flow through an associated one of the display elements is detected through a serial resistor; the resultant signal is subjected to A.C. amplification through the capacitor coupling; a difference between a peak value and a bottom value of the signal is obtained by an A/D conversion circuit to obtain the magnitude of a driving current; the driving current is multiplied by a drive ON time to obtain the supplied electric charges; and at a time point when the amount of supplied electric charges has reached a specified value, the circuit a is turned OFF. In this configuration, since the A.C. amplification does not need to be employed, the accurate current can be measured using a simple amplification circuit without any adjustments, which also contributes to the enhancement of the yield of the circuits.
In addition, at the rate of about 0.05 seconds per minute, the operation state is changed from the normal display state over to the operation mode of measuring the characteristics to carry out the current measurement; on the basis of this measurement result, a proper drive correction value is obtained; and the correction value is accumulated which is in turn read out if necessary to control the duty ratio of ON/OFF of the constant voltage drive.
Further in the operation mode of measuring the characteristics, the control made by the associated one of driving FETS is changed in an analog manner; in correspondence thereto, the current and the voltage of the associated one of the display elements are both measured; on the basis of this measurement result, a proper drive correction value is obtained; and the correction value is accumulated which is in turn read out if necessary to control the duty ratio of ON/OFF of the constant voltage drive and to detect the failure of any of the associated one of the display elements to generate a signal exhibiting the abnormality.
In addition, the operation mode of the driving FET for the associated one of the display elements is changed from the ON/OFF control for the normal display over to the constant current drive in the operation mode of measuring the V-I characteristics to subject the voltage of the associated one of the display elements to the A/D conversion, whereby it is also possible to obtain the V-I characteristics. In this configuration, though the current is not directly measured, a relatively simple circuit configuration can be obtained.